The present invention generally relates to a method for preparing titanium and titanium-alloy articles and more specifically to a method for preparing ultra-fine grain titanium and titanium-alloy articles and articles prepared thereby.
Currently, in the fabrication of titanium and titanium-alloy articles, thermal or heat-treating processes are included in the manufacturing process. These steps are to ensure that material grain size is produced and maintained at a level that is as small as possible. As such, it is the normal practice to employ a full annealing, i.e. recrystallization, or at least stress-relieving heat treatment steps in conjunction with any cold work or forming performed on the material. There have been exhaustive attempts to eliminate these thermal or heat-treating manufacturing process steps, which can account for up to approximately 20% of the costs associated with producing a titanium or titanium-alloy article such as a fastener, a rivet, a lockbolt or a threaded pin.
The grain size of a material formed is critical to both its ductility and strength among other properties. In general, grain sizes larger that ASTM 3 are not desirable for most cold-work or cold-forming operations. A duplexed grain size, defined as a significant difference in grain size depending upon location, should be avoided. Grain size is of special importance and generally increases in the degree of importance as the material is formed or mechanically-deformed to larger levels. As a rule, the finer the grain, the better the resulting formability. Recent research by Gysler et al. on xe2x80x9cInfluence of Grain Size on the Ductility of Age-Hardened Titanium Alloysxe2x80x9d and Thomas et al. on xe2x80x9cFriction Stir But Welding,xe2x80x9d which are herein incorporated by reference, have documented the directly proportional relationship between smaller grain size and improved material properties in titanium and titanium-alloy materials.
Friction stir welding (xe2x80x9cFSWxe2x80x9d) or more generally, friction stir processing (xe2x80x9cFSPxe2x80x9d), is a solid-state process that uses a nonconsumable tool to join various types of metals. When a FSP rotating tool is inserted into and traverses through the materials, the tool plasticizes the materials and forces the materials to flow around the tool where they reconsolidate. As has been demonstrated with aluminum-alloy materials, FSP produces ultra-fine grain material structures in the xe2x80x9cnuggetxe2x80x9d area of the processed material. From tests on aluminum-alloy materials, it has been determined that this resulting nugget material has shown improved material properties, the majority of which are directly dependent on grain size.
It is thus highly desirable utilize the FSP technology to form titanium or titanium-alloy articles having ultra fine grain metallurgical structures without a subsequent thermal or heat-treating processing step.
The present invention utilizes an FSP process to form a titanium or titanium-alloy article thereby producing a material with reduced grain size as well as improved homogeneity. This generates material structure having improved properties without the use of subsequent thermal or heat-treating procedures. Consequently, the overall manufacturing process for titanium or titanium-alloy articles such as fasteners can be shortened, thereby reducing manufacturing costs and eliminating the possibility of fasteners being improperly heat-treated.
Also, associated improved mechanical properties such as ductility and fracture toughness may be realized as a result of the resultant ultra-fine grain metallurgical structure within the article produced by the FSP as compared with articles produced using the prior or current art manufacturing processes. This could lead to substantial cost reduction depending upon the application of the articles produced using this process. For example, fasteners made according to this new process used in the aerospace industry could be reduced in size and still have the same mechanical properties. Further, these increased mechanical properties could lead to an overall reduction in the quantity of fasteners needed to secure together or assemble detail components. Both quantity and reduced size of fasteners could thus lead to further increased cost and labor savings as compared with the prior art.